Ion-exchange membranes have been industrially used in many fields as electrolytic membranes for conducting the electrodialysis in the step of desalting for producing salts and foods, in the fuel cells, and as membranes for diffusion dialysis for recovering acids from the acids that contain metal ions generated in the steel industries. The ion-exchange membranes have a structure in which a base sheet that serves as a reinforcing material is provided as a core material in the ion-exchange resin so as to impart a predetermined membrane strength and shape stability for the membranes. Without the core material, the ion-exchange membrane, due to its much ion-exchange groups, easily undergoes the swelling if it is dipped in an electrolytic aqueous solution, and loses the strength or is deformed.
There has heretofore been known an ion-exchange membrane using a porous resin sheet as the base sheet. In the ion-exchange membrane of this form, voids in the porous resin sheet which is the base material are filled with the ion-exchange resin offering an advantage of a small electric resistance of the membrane (hereinafter membrane resistance). As the porous resin sheet, there has, usually, been used a thin film of a polytetrafluoroethylene or a high molecular polyethylene resin, the thin film being stretched in a monoaxial direction or in biaxial directions to avoid a decrease in the strength caused by the porosity. For example, a patent document 1 discloses a cation-exchange membrane for producing salts that includes a porous stretched polyethylene sheet (HIPORE manufactured by Asahi Kasei Chemicals Co. or SETELA manufactured by Tonen-Kagaku-Nasu Co.) as the base sheet.